Exact response pattern
450 4.1.8 Sender address rejected: Domain not foundTemporaryGenericDNS
What it means
The sender domain could not be resolved at the time of delivery.
Likely causes
- Temporary DNS failure.
- Missing or recently changed DNS.
Recommended response
- Verify envelope sender DNS.
- Retry after DNS is fixed and propagated.
Operational rule: Preserve the complete SMTP response, remote hostname, sending IP, envelope sender and timestamp. Never diagnose from the three-digit code alone.